feat(offline): auto-detect no network at cold start + 'go offline?' dialog

A sticky-online cold start with no network hung the splash on adoptSession's
retrying profile fetch. Now, for an offline-capable web install with a cached
profile:
- No network interface (navigator.onLine === false) -> enter offline mode for
  the session (no dialog; the next launch re-evaluates).
- Interface up but the gateway is unreachable within 3s (a single-attempt
  reachability probe, not the 6-retry loop) -> a 'No connection. Enable offline
  mode?' dialog: Enable -> sticky offline; Keep trying -> the normal online
  adopt (retries, 'Connecting...').

- connection.svelte: checkReachable(timeout) - a bounded single probe.
- offline.svelte: setOfflineMode(on, persist) - auto-offline is session-only, a
  deliberate choice (dialog/toggle) is sticky.
- app.svelte.ts: the cold-start auto-detect in bootstrap + the dialog resolver;
  App.svelte renders the boot dialog. i18n en/ru.
- App-entry bundle budget 113->114 (the boot path cannot be lazy-loaded).

Online cold-start unaffected (auto-detect gated to isStandalone, off in the mock
e2e): e2e 196. The offline paths are contour-verified.

Next: PR2 - mid-session flight-mode reactivity (online/offline events).

ui/scripts/bundle-size.mjs

@@ -20,12 +20,12 @@ import { join } from 'node:path';
 const DIST = 'dist';
 
 // Per-chunk gzip budgets in KB. The app entry was raised to 110 for the local move-preview
-// wiring, to 112 for the PWA install feature, then to 113 for the offline-mode wiring: the
-// dictionary-preload trigger and the offline-state reactives live in the app entry (the Header
-// reads them, the lobby/profile screens fire the trigger), while the heavy parts — the dict
-// loader, the move generator and the preload orchestration — stay in lazy chunks. Scoped CSS
-// lands in the CSS chunk, not this JS budget.
-const BUDGET = { app: 113, shared: 30, landing: 5 };
+// wiring, to 112 for the PWA install feature, to 113 for the offline-mode wiring, then to 114 for
+// the offline auto-detect: the cold-start reachability check and the "no connection" dialog live in
+// the boot path (app.svelte.ts / the App shell), which cannot be lazy-loaded. The heavy parts — the
+// dict loader, the move generator and the preload orchestration — still stay in lazy chunks. Scoped
+// CSS lands in the CSS chunk, not this JS budget.
+const BUDGET = { app: 114, shared: 30, landing: 5 };
 
 // gzipped returns the gzipped byte size of a built asset, or 0 when the reference is not a
 // local file (e.g. the Telegram SDK loaded from a CDN) or is missing.